1. Field of the Invention
This invention relates to controllers for video recorders. In particular, this invention relates to methods and apparatuses which control the recording of single fields of video information on separate tracks of a magnetic disc drive storage unit.
2. Description of the Prior Art
The medical profession has for some time used a machine known as a fluoroscope to examine internal structures. In such a machine, X-rays are directed through objects or parts onto a fluorescent screen. The result is the appearance of shadows of the objects or parts on the fluorescent screen. Although of tremendous diagnostic value, prolonged exposure to X-rays is now known to be highly undesirable.
One known technique of retaining the benefits of fluoroscopy while minimizing X-ray exposure is to momentarily pulse the X-ray generator and then to photograph the resulting momentary image. While X-ray films have been widely used for this purpose, a more flexible and useful technique is to use an image intensifier in place of the fluoroscope screen and to photograph the momentary image formed on the image intensifier with an electronic television camera connected to a video recorder. This process is known as electronic radiography.
Although inexpensive, magnetic tape video recorders are poorly suited for this application. Not only is it difficult for the operator to locate the single frame of recorded information, but the magnetic tape will not survive the prolonged repeated viewing of the screen single frame which is necessary to produce a still image on the television screen.
One solution to this problem which applicant has employed in the past is to record the single frame of video information in analog form on a single track of a magnetic disc drive. The frame of video information contained on the track of the disc drive is then repeatedly played back onto a TV monitor screen thus recreating the still picture which previously appeared on the fluoroscope screen only momentarily.
Unlike X-ray film, applicant's video recording system can also be used "on line". In this mode, the fluoroscope is pulsed at a rapid rate while the video recorder is synchronized to update the picture viewed on the video monitor with the last picture displayed on the fluoroscope. In this mode, the interior of the X-rayed object can be effectively monitored continuously with only a fraction of the X-ray exposure which an ordinary fluoroscope would otherwise require.
There are many applications, however, where updating is not required very frequently. In such a case, it would be beneficial to provide slower updating speeds so as to further minimize unnecessary x-ray exposure.
To accomplish this result, applicant has in the past utilized a separate delay timer to inhibit updating of the monitor screen (and firing of the fluoroscope) for pre-determined intervals of time. Usage of a separate delay timer, however, requires that the delay timer be synchronized to the video frame information to insure that the delay timer will not signal the magnetic disc drive unit to begin recording a new video picture frame at any time other than the beginning of the new video picture frame. However, not only is the expense of such additional circuitry undesirable, but applicant has experienced difficulty in making it work properly.
The need for synchronization also exists when it is desired to sequentially play back a series of separately recorded tracks of recorder video frame information. In order to avoid tearing the picture when the access arm carrying the read-write heads on the disc drive is repositioned over a different track, it is necessary to synchronize the movement of the access arm so that it passes over guard bands separating recorded tracks of video information only during the vertical interval of the video frame. Due to the mechanical delays inherent in moving the access arm, however, to achieve this result it is necessary to commence movement of the access arm from one track to another well before the vertical interval actually begins. To achieve this result, applicant has in the past used a separate single anticipation timer which, upon receiving the appropriate command, causes the access arm to commence movement to a new track a short period of time prior to commencement of the vertical interval.
The amount of anticipatory time required for the anticipation timer is dependent upon the intrinsic mechanical and electrical delays of the system. Although applicant has in the past found it necessary to individually set the amount of anticipatory time for each unit, even with such detailed adjustment, tearing of the monitored picture still occasionally occurs during movement of the access arm. This tearing is highly undesirable because it disrupts the doctor's concentration, and is especially objectionable when the system is changing tracks at a relatively rapid rate.